It may be that one or both of your stanchions are bent, causing them to catch as they go up and down the slider. This may not be obvious when looking at it from the front depending how they are oriented in the trees. It doesn't take much of a bend to cause this, had it myself, and the only real way to tell is to dismantle and roll them across a flat surface, like checking a pool cue. An incorrect spacer may also cause this problem, pulling the slider in toward the wheel and away from the perpendicular.
You could also try centralising the front wheel, if you haven't done that already. Loosen the wheel spindle and give the front end a few pumps so you know the sliders are locating correctly. Then re-tighten the spindle with the front brake on. This is at least the correct way to fit the front wheel on my old Brit drum-equipped front brake, I assume it would also be the same for disc brakes and I tend to always do it this way.
